The invention relates to a catadioptric projection lens for imaging a pattern arranged in an object plane onto an image plane.
Projection lenses of that type are employed on projection exposure systems, in particular wafer scanners or wafer steppers, used for fabricating semiconductor devices and other types of microdevices and serve to project patterns on photomasks or reticles, hereinafter referred to as “masks” or “reticles,” onto an object having a photosensitive coating with ultrahigh resolution on a reduced scale.
In order create even finer structures, it is desired to both increase the image-end numerical aperture (NA) of the projection lens and to employ shorter wavelengths, preferably ultraviolet light with wavelengths less than about 260 nm.
However, there are very few materials, in particular, synthetic quartz glass and crystalline fluorides, such as calcium fluoride, barium fluoride, magnesium fluoride, lithium-calcium-aluminum fluoride, lithium-strontium-aluminum fluoride, lithium fluoride, and similar, that are sufficiently transparent in that wavelength region available for fabricating the optical elements required. Since the Abbé numbers of those materials that are available lie rather close to one another, it is difficult to provide purely refractive systems that are sufficiently well color-corrected (corrected for chromatic aberrations). Although this problem might well be solved by employing purely reflective systems, fabricating such mirror systems involves substantial expense and effort.
In view of the aforementioned problems, catadioptric systems that combine refracting and reflecting elements, in particular, lenses and mirrors, are usually employed for configuring high-resolution projection lenses of the aforementioned type.
Whenever imaging reflective surfaces are involved, it is beneficial to employ beam-deflecting devices if images free of obscurations and vignetting are to be achieved. Both systems having geometric beamsplitters and systems having physical beamsplitters are known. A system having geometric beamsplitting achieved with the aid of a pair of beam-deflecting mirrors is disclosed in European Patent EP 0 989 434, which corresponds to U.S. Ser. No. 09/364,382. Whereas systems having geometric beamsplitting have the disadvantage that they are necessarily off-axis systems due to the geometric beamsplitting, employing a physical beamsplitter will allow configuring on-axis systems.
A system having a physical beamsplitter is known from European Patent EP-A-0 475 020, which corresponds to U.S. Pat. No. 5,052,763. That system has at least one catadioptric entrance system and a dioptric exit system. The mask to be imaged lies directly on a beamsplitter that is configured in the form of a beamsplitter cube (BSC) and deflects part of the light reflected by the catadioptric system to the dioptric system. However, arranging the object to be imaged directly on the beamsplitter limits opportunities for correcting the entire system. Moreover, the object end of this system is nontelecentric.
Another catadioptric system having a physical beamsplitter and an intermediate image is known from U.S. Pat. No. 5,694,241, where a lens group having a positive refractive power is arranged between the object plane and the beamsplitter, which is remote from the object plane.
A catadioptric projection lens that has no intermediate image wherein a first lens group is provided between the object plane and a physical beamsplitter, a second lens group is provided between the physical beamsplitter and a concave mirror, and a third lens group is provided between the physical beamsplitter and the image plane is known from European Patent Application EP-A-0 350 955, which corresponds to U.S. patent application U.S. Pat. No. 4,953,960. The lens group arranged between the beamsplitter and the concave mirror are intended to correct for low-order coma and spherical aberrations of the concave mirror, as well as Gaussian errors, where a low negative refractive power will be sufficient to compensate for longitudinal chromatic aberration of the refractive groups.
A catadioptric projection lens that has a physical beamsplitter and no intermediate image and allows image-end high numerical apertures of at least 0.5, combined with a beneficial design and low sensitivity to alignment errors is Known from German Patent Application DE-A-42 03 464, which corresponds to U.S. patent application U.S. Pat. No. 5,402,267. This system is largely characterized by the fact that no lens group is arranged between the concave mirror and beamsplitter, and that the concave mirror has a strong reduction effect, i.e., a strong image demagnification. Longitudinal chromatic aberration (CHL) is primarily corrected by employing a highly convergent beam in the beamsplitter cube and may result in full achromatization of longitudinal chromatic aberration. The beam ahead of the concave mirror, i.e., that undergoing its first pass of the beamsplitter, is nearly, or substantially, collimated, while the beam following the concave mirror, i.e., that undergoing its second pass of the beamsplitter, is normally highly convergent. The system stop is preferably situated at the concave mirror and defined by its perimeter. This stop may also be defined on the surface of the beamsplitter facing the concave mirror or by inserting a stop between the concave mirror and the beamsplitter. Other benefits provided by the highly convergent beam following reflection at the concave mirror are that only a slight amount of refractive power must be provided following the beamsplitter and that relatively small beam heights occur in that vicinity, which means that adverse effects on chromatic aberration due to large beam heights in that vicinity are avoided.
The benefits provided by lenses of that type are counteracted by a disadvantage, namely that the beam incident on the beamsplitting surface is convergent, particularly on the second pass, following reflection at the concave mirror, which means that the range of incidence angles occurring at its surface is very broad, which imposes stringent demands on the quality of its beamsplitting coating. The sharp convergence of the beam following reflection at the concave mirror causes also means that very little space is available for lenses following the beamsplitter and thus that few means for correcting for aberrations remain available. Further increasing image-end numerical aperture would both necessitate employing a larger beamsplitter cube and shift the image plane closer to the beamsplitter. Such projection lenses are thus also regarded as “aperture-limited.” Another of their disadvantages is that they require employing relatively large beamsplitters, which are expensive, due to the limited availability of materials suitable for their fabrication.
Fundamentally similar problems also arise in the case of projection lenses that are configured in fashions similar to, and have beam paths similar to, that depicted in German Patent Application DE-A-42 03 464. Included thereunder are, for example, those projection lenses depicted in U.S. patent applications U.S. Pat. No. 6,118,596, U.S. Pat. No. 6,108,140, and U.S. Pat. No. 6,101,047. Large incidence angles on beamsplitting surfaces may also occur in the case of systems, such as those depicted in U.S. Patents U.S. Pat. No. 5,808,805, U.S. Pat. No. 5,999,333, or U.S. Pat. No. 5,861,997, where an intermediate image is created in the vicinity of their beamsplitting surface.
A catadioptric projection lens that has a physical beamsplitter and no intermediate image, wherein the beam is slightly divergent on its first pass of the beamsplitting coating and is collimated on its second pass of that coating, i.e., following reflection at the concave mirror, which is intended to allow avoiding deteriorations in image quality due to the dependence of the reflectance of the beamsplitting coating on angle of incidence, is known from U.S. patent application U.S. Pat. No. 5,771,125. The light reflected by that coating is collimated by keeping the refractive power of the lens group that includes the concave mirror relatively low. However, in the case of the system depicted in European Patent Application EP-A-0 602 923, which corresponds to U.S. patent application U.S. Pat. No. 5,715,084, a positive lens is arranged ahead of the physical beamsplitter in order to collimate the beam incident on the beamsplitting coating on its first pass of that coating. The beam is convergent following reflection at the concave mirror. Such systems require use of large beamsplitters.
In the case of a catadioptric projection system having a physical beamsplitter and no intermediate image, German Patent Application DE-A-44 17 489, which corresponds to U.S. patent application U.S. Pat. No. 5,742,436, proposes arranging at least one condenser lens, i.e. a lens with postive refractive power for collimating the beam incident on the beamsplitting coating on its object end, ahead of the physical beamsplitter, in order to minimize the incidence angles on the beamsplitting coating, and a scattering lens group having a negative lens following the physical beamsplitter in its catadioptric lens section, i.e., between the beamsplitter and the concave mirror, in order to compensate for the effect of the condenser lens ahead of the beamsplitter and correct for longitudinal chromatic aberration. Under this design, the beam is substantially collimated for both directions of propagation through the beamsplitter cube. The system stop normally follows the beamsplitter cube in the optical train. Since the beam is substantially collimated for both directions of propagation through the beamsplitter cube, problems that arise for broad ranges of incidence angles thereon are avoided. Another benefit of this collimated beam on the second pass, following reflection at the concave mirror, is that sufficient space remains available on the image side of the beamsplitter for incorporating means for correcting for aberrations. The disadvantage of the arrangement in accordance with German Patent Application DE-A-44 17 489 is that its overcorrected catadioptric lens section is incapable of fully correcting for longitudinal chromatic aberration (CHL). That arrangement also requires employment of relatively large beamsplitters, which is a disadvantage due to the limited availability of materials suitable for their fabrication.
European Patent EP 1 102 100, which corresponds to U.S. Ser. No. 09/711,256, describes a catadioptric reduction lens that differs from the design depicted in German Patent DE 44 17 489 in that, among other things, it allows fully correcting for longitudinal chromatic aberration. That system has a first lens section having a positive refractive power, a physical beamsplitter having a beamsplitting surface, a second lens section that is arranged between the physical beamsplitter and a concave lens, and a third lens section that is arranged between the beamsplitter and its image plane, between its object plane and its image plane, in that order. At least two lenses having negative refractive powers re arranged within its second lens section. That concentration of high negative refractive power, in particular, in the vicinity of the concave mirror, benefits fully correcting for longitudinal chromatic aberration and allows configuring systems that have working distances, both between their object plane and their first lens section and between their third lens section and their image plane, that are sufficiently large to allow their employment in microlithographic applications Such systems also allow providing a largely collimated beam in the vicinity of their beamsplitting surface for both directions of propagation through their beamsplitting surface in order that the angles incidence on their beamsplitting surface will be confined to a narrow range, which will allow largely avoiding the aforementioned disadvantages of widely varying angles of incidence. That patent describes embodiments having an intermediate image, combined with numerical apertures, NA, ranging up to about NA=0.7. The disadvantage involved there is that relatively large quantities of materials that, in suitable qualities, are in short supply, are required for fabricating their beamsplitters.
Among other things, U.S. Pat. No. 6,377,338 B1 describes systems having physical beamsplitters fabricated from crystalline fluoride materials.
In the case of the aforementioned systems having physical beamsplitters, their respective beamsplitting surfaces are inclined at an angle of 45° with respect to a segment of their optical axis orthogonal to their image plane. Any deflecting or folding mirrors that may be present, in particular, in their second lens section, are also inclined at an angle of 45° with respect to their optical axis in order to fold their optical axis through a right angle, which allows achieving an orthogonal or parallel arrangement of their object plane and image plane, whichever may be preferred, where the latter arrangement is particularly beneficial for scanner operation.